Why did the Germans and the French use trenches during the war?

They used trenches to stay ducked and covered and to protect soldiers from poisonous gas and gunshots

Long, narrow trenches dug into the ground at the front, usually by the infantry soldiers who would occupy them for weeks at a time, were designed to protect World War I troops from machine-gun fire and artillery attack from the air. Trenches may have afforded some protection by allowing soldiers more time to take other defensive steps, such as putting on gas masks